Morphological study of Czochralski-grown lanthanide orthovanadate single crystals and implications on the mechanism of bulk spiral formation

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2010Hengjiang Cong
Single crystals of monoclinic Nd:LaVO4 with dimensions up to Ø28 × 21,mm have been grown from the near-stoichiometric melt by the Czochralski method, making use of various seed orientations that are perpendicular to the (010), (10), (001) and (00) crystal planes.
A sample was also prepared with the seed orientation in an arbitrary direction relative to the crystal.
The anisotropic properties of the crystal are manifested in the growth morphology of the as-grown crystals, where different degrees of bulk spiral growth were observed.
It was also found that employing the (001) or (00) seed faces severely suppressed the bulk spiral growth, and thus high quality and large-scale Nd:LaVO4 crystals were obtained.
The constituent segregation coefficients and high-temperature stability, including the melting point, were determined and evaluated.
Based on the attachment energy model of Hartman,Perdok theory, morphology predictions were made for monoclinic LaVO4 and tetragonal YVO4 orthovanadate single crystals.
Correlating with the as-grown morphology of both crystals developed along different seed orientations, a theoretical explanation is provided for the influences of seed crystals on bulk spiral formation, crystal quality and utilization ratio.
It suggests that breaking the axial symmetry of the ideal atomic level interface between crystal and melt plays a crucial triggering role in bulk spiral formation in the Czochralski growth of lanthanide orthovanadate single crystals.
Selecting a proper seed orientation that yields such a highly axially symmetric surface structure consisting of a series of large-area facets with similar growth velocities can greatly reduce bulk spiral formation and thus is preferable in the Czochralski growth of large-sized low-symmetry oxide crystals.
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ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2000-K3NdSi6O15·2H2O, its polymorphs
Hydrothermally grown crystals of ,-K3NdSi6O15·2H2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods.
The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008,(2), b = 15.004,(2) and c = 7.2794,(7),Å, giving a calculated density of 2.683,Mg,m,3.
Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules.
The structure is based on highly corrugated (Si2O52,), layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains.
The silicate layers are connected by Nd octahedra to form a three-dimensional framework.
Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001].
Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double.
In addition, two phase transitions were found to occur upon heating.
The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact.
Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport.
Ion-exchange experiments revealed that silver can readily be incorporated into the structure.
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